1. Field of the Invention
The present invention relates to a method of manufacturing a gallium nitride-based (AlxInyGa(1−x−y)N, where 0≦x≦1, 0≦y≦1, 0≦x+y≦1) single crystal substrate, and more particularly to a method of manufacturing a gallium nitride (GaN) single crystal substrate using a ZnO substrate.
2. Description of the Related Art
Recently, as a higher density and a higher resolution in recording/reproduction are increasingly required in the field of an optical disk, investigations have been actively undertaken on a semiconductor device capable of emitting light in a short wavelength range. As a material for the semiconductor device capable of emitting light in the short wavelength range, a gallium nitride single crystal substrate has been widely used. Since the gallium nitride single crystal has the energy band gap of 3.39 eV, the material is adequate for emitting the light in a short wavelength range of blue light of the visible spectrum.
Conventionally, the gallium nitride single crystal can be grown on a dissimilar substrate using the MBE (Molecular Beam Epitaxy) process or a vapor phase growing process, such as the MOCVD (Metal Organic Chemical Vapor Deposition) process, the HVPE (Hydride Vapor Phase Epitaxy) process, etc. As the dissimilar substrate to be used in these processes, a sapphire (α-Al2O3) substrate or a SiC substrate is prepared. In particular, due to its hexagonal lattice structure, the same as that of gallium nitride, its lower price compared with the SiC substrate, and its high temperature stability, the sapphire substrate is generally used.
However, due to a difference of about 13% in lattice parameters and a high difference of −34% in thermal expansion coefficients between the sapphire substrate and the gallium nitride, there is a problem that strains can be created at the interface between the sapphire substrate and the GaN single crystal, generating lattice defects and cracks in the crystal. Such lattice defects and cracks make a growth of a high quality gallium nitride-based single crystal difficult, and a device made of the single crystal with such lattice defects has a short life span.
In order to solve these problems, a freestanding gallium nitride-based single crystal substrate is required. In a conventional method, the freestanding gallium nitride-based single crystal substrate is provided by course of primarily growing GaN single crystal bulk on the sapphire substrate and removing the sapphire substrate from the GaN-based single crystal bulk. Here, as methods for removing the sapphire substrate, a mechanical process using diamond powder, a chemical etching process, etc. can be applied.
In case of the mechanical process, since stresses applied to the sapphire substrate in the grown state of the GaN-based single crystal bulk are within the elastic limit thereof, the stresses bend the substrate without generating the cracks. As the mechanical process progresses, however, the sapphire substrate becomes thin so that balances between the forces are lost, creating the cracks on the sapphire substrate, which will propagate to the gallium nitride film, also resulting in the cracks on the gallium nitride film.
Additionally, in case of the chemical etching process, it is not easy to prepare etchant which can selectively etch only the sapphire substrate with a high etching rate.
Recently, there has been used a laser lift-off process, by which after being decomposed into metallic Ga and nitrogen (½N2) at the interface with the sapphire substrate by irradiating the laser, the GaN-based single crystal bulk is melted and separated from the sapphire substrate at a high temperature. Although this process can be applied without creating the cracks on the substrate having a small size, there is a problem that the cracks are still created when the process is applied to a wafer of 2 inches (2″) or more in diameter necessary for a process of manufacturing the semiconductor. That is, as shown in FIG. 1, when irradiating the sapphire substrate 11 for growth with the laser beam, it is conducted in such a manner that a local irradiation is repeated on the sapphire substrate 11, several times, due to narrow irradiation areas of the laser beam (to the extent of 10 mm×10 mm at most, until now), whereby stresses due to the lattice and thermal coefficient mismatch can become more serious.
By this, the cracks occur in the grown GaN-based single crystal bulk 15 and propagate along cleavage planes resulting in fracture of the GaN singly crystal bulk 15.
Thus, there is a need in the art to provide a new method of manufacturing a high quality GaN-based single crystal substrate by thoroughly solving the problem of the stresses caused by the differences in lattice parameters and in crystal structure between the GaN-based single crystal bulk and the prior substrate for growth, such as the sapphire substrate.